Red iron oxides are commonly used as pigments for colouring of construction materials, paints and coatings, plastics, paper, cosmetics and the like. In addition they may be used as catalysts in chemical reactions such as conversion of ethylbenzene to styrene and as burn rate control agents for solid rocket fuel. For these applications products typically have a small (0.1 micron to 3 micron particle size and preferably consist of pure hematite crystals with minimal content of other iron oxide crystal types.
Most of the processes commonly used to produce red iron oxides employ iron sulphate as starting material. However, iron sulphate feedstock is becoming increasingly difficult and expensive to obtain due to closure of titanium dioxide plants based on sulphate technology, reduction of steel pickling operations using sulphuric acid in Western Europe and the U.S.A., and increased use of ferrous sulphate as a reductive for chromium (VI) in the cement industry. The alternative of dissolving scrap steel in sulphuric acid is becoming increasingly costly as well and necessitates design of reactors to manage the hydrogen liberated.
In addition, environmental concerns are related to the red iron oxides production processes based on iron sulphate. During annealing of iron sulphate, sulphur gases are released which must be collected and converted back to sulphuric acid in a sulphuric acid plant while not reacted iron sulphate in the product is typically neutralized to give a gypsum waste stream. In precipitation processes a salt solution is generated which must be treated before release to the environment, typically by evaporation to sodium or ammonium sulphate. Moreover, red iron oxides obtained from sulphate precipitation processes usually contain significant quantities of goethite which reduces the brightness and colouring strength of the hematite pigment.
In contrast to ferrous sulphate solutions, ferrous chloride solutions are more readily available, for example, from pickling of steel or beneficiation of ilmenite ores by acid leaching processes. Typically, ferrous chloride pickling wastes have been disposed by roasting to recycle the hydrochloric acid value while generating a very low value iron oxide by-product. Alternately, ferrous chloride pickling wastes are neutralized and disposed in a landfill. However, the ferrous chloride by-products contain a significant level of impurities such as manganese and zinc, which can have negative effects on pigment production processes.
French patent 1498479 describes a process for precipitation of an iron hydroxide gel or goethite from a ferric salt solution and subsequent thermal treatment under hydrothermal conditions (above 100° C.), typically above 120° C. and up to 250° C. to transform this gel to ferric oxide.
A process for production of red iron oxide pigments from ferrous chloride solutions with the added advantage of calcium chloride recovery is described in Indian patent 174841. The process involves neutralization of the ferrous chloride with calcium hydroxide followed by oxidation over 4 to 6 hours, separation of the calcium chloride solution for recovery, washing, drying and calcining the product iron oxide at 750° C. to 800° C. to achieve the target iron oxide.
German patent 1040155 discloses a process for precipitation of red iron oxides without a calcination step. Metallic iron is treated with oxygen containing gases in an aqueous iron (II) salt solution containing seeds of iron oxide or iron oxide hydroxide, wherein the seeds are prepared from precipitation of iron (II) ions by addition of alkali or alkaline earth hydroxides or carbonates. Both ferrous sulphate and ferrous chloride are suitable.
Methods for producing a precipitated red iron oxide pigment having low goethite contents are described in U.S. Pat. No. 3,946,103, wherein a product with a goethite content of less than 15% is obtained.
U.S. Pat. No. 7,144,455 relates to a method for precipitating yellow iron oxide pigments (goethite) from ferrous chloride solutions and their use in preparing red iron oxide pigments by calcination.
There remains a need for cost effective production methods for red iron oxide pigments having a low goethite content or being essentially free of goethite.